unfortunate that 4800 wasn’t stable but it really does look like there could be a 4600 brick wall. Mine fails TM5 after about an hour at 4800.
Even if we started getting 4800 stable by turning off the igpu or playing with more settings, I think in some conditions someone could be getting memory errors without knowing. At least we know because 4800 is barely unstable that its likely 4600 is fairly rock solid for us.
You could up your tREFI and claw back some of the performance from dropping from 4800 to 4600 I think too.
Maybe the next round of BIOS updates will have 2T command mode or some other memory related tweaks and we can try for 4800+